The present invention relates to a system for controlling air-fuel ratio for an internal combustion engine emission control system with a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio in an unusual condition such as the malfunction of the carburetor.
Such a control system is a feedback control system, in which an oxygen sensor is provided to sense the oxygen concentration of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the burned air-fuel mixture.
The oxygen sensor generates a high voltage when the air-fuel ratio of the exhaust gases is smaller than the stoichiometric air-fuel ratio and generates a low voltage when the air-fuel ratio is greater than the stoichiometric ratio. The control system operates to correct the air-fuel ratio given by the carburetor to the stoichiometric air-fuel ratio in dependency upon the output voltage of the oxygen sensor. In such a control system, various control correction means are provided for fixing the air-fuel ratio to a predetermined constant value during an unusual condition. For example, if the idling operation of the engine continues for a long time, the temperature of the exhaust gases decreases, which causes a decrease in the temperature of the oxygen sensor body. When the temperature of the oxygen sensor body decreases, the output voltage of the sensor decreases. The voltage decrease is the same as when the air-fuel ratio of the exhaust gases becomes greater than the stoichiometric ratio. Thus, the feedback control system operates to actuate an air-fuel ratio correcting means, such as an electromagnetic valve, to correct the air-fuel ratio to a smaller air-fuel ratio. But such a correcting operation is also performed when the carburetor supplies a rich or stoichiometric air-fuel ratio mixture. As a result, the mixture induced in the engine is excessively enriched.
In order to prevent such as excessive enrichment of the mixture, the feedback system is constructed so as to actuate the air-fuel ratio correcting means at a predetermined constant duty ratio when an enrichment correction operation having a duty ratio greater than a predetermined ratio continues for a predetermined period. However, in such a system, if the carburetor malfunctions to supply a very lean mixture, the control system continues to enrich the air-fuel mixture at the minimum duty ratio. And if the enrichment control operation continues for the predetermined period, the feedback system is changed to the constant duty ratio, that is the predetermined greater duty ratio supply condition. As a result, a much leaner mixture is supplied. Such a lean mixture can cause malfunctioning of the engine.